In my prior U.S. Pat. No. 3,725,967 I have disclosed a mechanism of this type, used in an apparatus for washing automotive vehicles or the like, in which a cylindrical brush with a horizontal shaft is suspended within a portal frame by cables carrying counterweights partly balancing the weight of the brush and allowing it to settle by gravity on a hood, top or trunk of a vehicle and to creep up a sloping front or rear surface thereof by virtue of its own rotation even as it cleanses that surface. The system of this prior patent includes separate electric motors for relatively displacing the portal frame and the vehicle to be washed, for rotating the horizontal brush, and for hoisting that brush through a slipping clutch coupled with its cable linkage whenever this becomes necessary to clear an obstacle. An electric hoist motor may also be used, as described in my prior U.S. Pat. No. 3,688,329, to elevate such a brush rotating in a sense opposing an upward climb.
Experience has shown that a scrubbing brush descending by gravity, even with its own weight partly compensated by a counterweight, may sometimes strike a vehicular surface with too much force. Conversely, the response of an electric hoist motor to a switch sensitive to the encounter of a steeply inclined surface --as described in my prior U.S. Pat. No. 3,725,967 --may result in such a fast ascent of the brush that the surface in contact therewith is insufficiently scrubbed, particularly when the brush is corotating to promote the aforedescribed upward creep in the unoperated state of the hoist motor.